Efficiency Study on Mercedes AMG F1 Engine
For the final project of my Renewables, Fuel Cells and Internal Combustion Engines class my group decided to do a study on how the Mercedes AMG Petronas F1 team was able to achieve a claimed 50% thermal efficiency out of their engines. This would require us to use all knowledge we have learned about applying the thermodynamic cycle, chemistry, Wiebe function, and engine components and their efficiency effects to approximate how the power unit was able to achieve the claimed 50% efficiency.
To start this process we used the FIA Engine regulations from the Turbo-Hybrid Era. These regulations outlined fuel flow limits, engine geometry, and fuel type. Then we paired this with overall Formula 1 knowledge of when most drivers shift to get maximum engine RPM, and the general claim of 1,000 brake horsepower.
Fuel Chemistry
Knowing the geometry and type of fuel that was used in the engine we then were able to calculate out all needed values of the fuel properties. In this set up we used Octane which we found had a lower heating value of 46 MJ/kg. Our chemistry sheet would calulate out the number of atom for carbon, hydrogen, oxygen, and nitrogen. From this and the sheet would calculate the stoichiometric equation for fuel and air ratio then also give Engine Specific CO2 as well as Brake Specific CO2. There were some needed values to be input to be able to get these numbers like burn efficiency of the fuel which we entered as .52. This number came from research finding the implementation of a combustion pre chamber can get approximately 50% burn efficiency due to its ability to increase burn speed from sending flame jets into the cylinder instead of a small spark to ignite the entire chambers fuel content.
Thermodynamic Properties of Cylinder
Motivation
To complete the feasibility of the engine we had to calculate the cylinder pressure during combustion. We took the claimed thermodynamic efficiency of the engine to be 50%, then took the geometry and fueling parameters for the engine, then finally the estimated power output. But none of these inputs deal with what stresses the engine components would experience, so this why we had to look at the cylinder pressure required to be able to get these outputs. The engine efficiency has direct correlation to the compression ratio but the high the compression ratio the higher the pressure would be inside the cylinder, so the limiting factor would be the engines ability to be reliable pushing very high internal pressures.
Final Results
Given all the parameters and controlling the heat release of the combustion event we were able to see 50% thermal efficiency. However the cylinder pressures required to do so were so astronomically improbable that we were unable to fully get the engine to operate at 50% with staying under 250 bar of cylinder pressure. Our max cylinder pressure with this result was over 500 bar. With lengthening the combustion even say changing the start to -10 degrees before top dead center and having the event end at 15 degrees after top dead center we were able to get the pressures under control however thermodynamic efficiency was only 35%.
In the end we were able to build a fully functional RFD tool that given engine parameters would give the calculated engine efficiency and the pressure traces of the engine throughout the cycle. We were not able to fully get the claimed 50% thermal efficiency while staying under 250 bar of pressure however, if we were I would imagine I would be approached by Mercedes themselves.
The biggest take away from this project was the ability to build this tool and take publicly available information and get see what changes to the engine parameters can do to the power able to be produced as well as the effect those changes make on the cylinder pressures seen. This project gave great insight into how each parameter of the engine and changing it will effect different outputs of the engine.
Engine Geometry
We took this information and began building a Rate of Force Development (RFD) table built in Excel. This table had all of the engine geometries like number of cylinders, bore, stroke, compression ratio, lower heating value of fuel, max RPM, all other needed parameters. This engine information would play into all other sheets used to calculate power later
Air and Fuel Requirements
Given the stoichiometric values for air to fuel ratio we then took these values and the know fuel flow FIA F1 maximum fuel flow regulations of 100 kg/hr and calculate the needed fuel shot per cycle. This was was the assumption that max power was made at 10,500 RPM based on when driver usually shift. These values were then able to calculate the need air flow. From what I understand, the power output of Formula 1 cars from this generation is regulated by the amount of fuel flow that can be used. This differs from a series like NASCAR which has regulations on air flow with the use of an air restrictor. Either way it limits a key component of the combustion process. In this case with the fuel being the limiting factor, the engine uses turbos to get the required air needed to get to lamba 1 for combustion for maximum power output from the given fuel shot per cycle. This is how we were able to get the air requirements for the engine given the fuel rate.
The air flow required for combustion is needed for the Wiebe function. This will give the amount of air, and therefore the volume and pressure of the that is going into the cylinder during the intake stroke.
Analysis
In order to compute the work completed by the engine we used the Wiebe heat release function. This involves calculating the finite volume of the engine at each crank position then calculating the pressure. The main concept is taking the position of the piston through its combustion cycle and tracking the pressure. Then the heat addition event (addition of fuel and sparking it) is controlled through spark timing. The heat addition from fuel is converted into pressure which drives the piston down. This is how the of the engine is calculated at each finite crank position. The sum of the work at each crank position gives the work of the engine. In this model we were able to control the rate at which heat was released and the spark timing and duration of burn. This is what controlled the Heat Release columns, with it starting at -10 degrees before top dead center and ending at 2.7 degrees after top dead center. These Heat Release Columns are what gave the Pressure with Heat Release Analysis columns the values they needed for the instantaneous heat release to calculate the cylinder pressure and therefore work.
